So, this is interesting.
You are right, currently there is basically no volume on the fiat markets, so basically if you want to convert wbtc or waves to fiat > then withdraw through the fiat gateways.. doesn't really make sense, as for now.

BUT, and this is a big but (not butt ahaha), there will be real businesses using waves as a gateway to the crypto world.
that's were the volume for those markets will come, in my opinion
What those businesses will be.... it's only speculation.
But they could bring a lot of volume and transactions... and we could even see a completely different situation on WavesDEX markets